The present invention relates to a suction box as defined in the preamble of claim 1.
In the following, both the invention and prior art are described mainly in connection with paper machines, yet without limiting the invention exclusively to paper machines. Therefore, the term `paper machine` in this application refers to paper or cardboard machines in general or machines used in pulp manufacture to produce webs containing cellulose fibers.
In paper machines, suction rolls are used for the dewatering of the web being produced, both after the head box and in the press section. A suction roll is a roller with perforations in the whole area of its jacket and with a suction box inside it, the suction box being attached or pressed to the interior surface of the suction roll, with a sealing between them. The purpose of the suction box is to suck water from the web and to draw air through the web to remove moisture from it with the air.
As the machines are operated at increasing speeds, i.e. with increasing speeds of rotation of the rolls, the operation of the suction rolls has become ineffective. A vacuum draws water into the perforations in the suction roll and somewhat into the roll, but the centrifugal force prevents most of the water from entering the suction box itself. The centrifugal force even returns some of the water back into the perforations in the suction rolls and to the outside of the suction roll. For this reason, the water removed from the web remains on the underside of the web, keeping it wet. After the suction stage, the water splashing out from inside the roll again wets the web or, in the press section, the felt. The perforations in the roll contain plenty of water rotating with the roll, and some of this water is also passed to the supply side of the web, wetting the web or the felt. Moreover, the water remaining in the perforations of the roll and the particles contained in the water tend to block the perforations, increasing the power requirement of the suction pumps.
In an attempt to enhance the operation of the suction rolls, the vacuum level in the suction box has been increased, which naturally means an increased power required by the vacuum pumps. This is about the only effect this measure has produced. The increase in the speed of the machines has been achieved by increasing the efficiency of the drying section or by increasing the machine length or the steam consumption.